Only to add to what's been posted already. A frugal Puppy is a modular build consisting of the following files:
initrd.(?z) -- which creates a RAM disk on bootup with instructions for managing the other files;
vmlinuz -- the kernel -- sort of like the engine to run other components
zdrv_xxx.sfs -- always contains the drivers compiled against the kernel which are one part of a 2-part system for communicating with a computer's hardware, i.e. monitor, mouse, keyboard, sound-card. May include firmware.
fdrv_xxx.sfs -- firmware, the second part of that system, but not kernel-specific so can be used with any kernel and is sometimes packaged/downloadable separately. [But, of course, included in an ISO].
Puppy_version_number.sfs -- the core applications (file-manager & window-manager), infra-structure, basic user applications and such goodies a Puppy's dev decides to include. It's contents are what distinguishes one Puppy from another.
Because of its modular build, it is possible --not often really necessary*-- to up-grade a Puppy's kernel (and zdrv) in order to obtain the latest security patches or to work with a new computer's hardware. It is also possible to 'down-grade' a Puppy's kernel & zdrv in order to work with an old computer's hardware. With Puppys published within the last 6 years +/- such 'kernel swap' can done manually in about 5 minutes. There's also an application which can shave a couple minutes of that procedure.
About the only applications which require updating are web-browsers, and this because web-browsers creators (e.g. mozille.org, google) frequently update to provide greater security and enhanced capabilities, and websites are increasingly denying access to other than recently published web-browsers. Despite such trend, Seamonkey 2.46 published about 5 years ago, can still provide satisfactory performance.
On this Section of forum, viewforum.php?f=90 you'll find various Web-browsers available for Puppys. Among them are 'portables' published by Mike Walsh and fredx181. Portables are just that: because they are not installed you can even use them with 'Full installed' Puppies.
-=-=-=-=-=--
* There are various reasons why a frugal-puppy is, OOTB, inherently safe; not the least of which is that (except for the things you install and preserve) it's core systems are in read-only files on storage. There have been no reports by any Puppy user of malware infestation. But, if you choose to, you can take additional steps to run an 'almost invulnerable Puppy', viewtopic.php?p=2235#p2235; or 'harden' web-browsers, viewtopic.php?f=90&t=1491